Optical discs, such as a CD (Compact Disc) and a DVD, are popularized. With respect to these optical discs, information is recorded onto the optical discs or the recorded information is read, by a laser irradiated from an optical pickup. Then, the recording operation and the reproduction operation at this time are performed by that the optical pickup is displaced along a track which is formed in a spiral shape on the optical disc, for example. Moreover, the laser irradiated from the optical pickup is focused on an information pit formed on the track, to thereby read the information content that is indicated by the information pit.
On the other hand, in some cases, there arises that the laser irradiated from the optical pickup is out of focus, i.e. defocus, during the recording or reproduction operation. Such defocus is not preferable from the viewpoint of appropriate information recording and reproduction. In particular, in the case of a multilayer type optical disc having multiple recording layers, there is such a disadvantage that the information is recorded into a recording layer which is different from a recording layer to be recorded originally. Or there is such a disadvantage that the information in a recording layer which is different from a recording layer to be reproduced originally is reproduced, or the like. In this case, there has been developed a technology in which the defocus is detected, for example, and improper jump between recording layers caused by the defocus and the direction of the improper jump are detected, thereby the recovery from the defocus is achieved.
Patent document 1: Japanese Patent Application Laying Open NO. 2000-09955